feat(stark-demo): add the ngx form error to the showcase

[nicanac]

ISSUES CLOSED: #1195

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

Add the ngx form error to the show case

[ ] Bugfix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number:#1195

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information